Adding staints to paintwall (two image textures) - UV map problem
So let say you want to make this object, but the sign is a PNG image apart (make sure it has a transparent background):



Load your background picture, create your first UV map, put your UV on the background.



Load your sign image, create a second UV map, put the face(s) you want on the sign, reduce the other ones so that they don't overlay the sign (I thought I could create a one face map but it seems like not, perhaps someone will tell):



In the Shader Editor, use this node chain. Use the Input > UV Map nodes to tell what UV map Blender is supposed to use for each image (you don't need to use the first UV Map node as the first UV map will be chosen by default). For the stain's Image Texture, choose Clip instead of Repeat to make sure that the other faces are not overlaying the sign (or move them over a transparent zone). Don't forget to plug the alpha output into the Factor of the Mix Shader.